Over the last decade, two significant advances have taken place in continuous cooking technology for the production of cellulosic pulp, primarily in the practice of the sulphate process, but also for use in the sulfite and other processes. First, with the advent of MCC.TM. digesters from Kamyr, Inc., a significant countercurrent cook zone was established in the middle of the digester by introducing white liquor into a central recirculation loop. Subsequently, EMCC.RTM. digesters developed by Kamyr, Inc., provided for further countercurrent cooking by introducing white liquor into the bottom (wash) circulation loop, as seen in copending application Ser. No. 07/583,043, filed Sep. 17, 1990 (now abandoned). These digesters have been commercially successful because they enhance the quality of the pulp produced. In both these commercial digesters, however, the first cooking zone is a co-current zone.
According to the present invention it has been found that if the entire cook in a continuous digester is made countercurrent, the temperature of the cook can be lowered, or more uniform and better temperature control can be provided. Lower temperatures or more uniform temperature control limits the damage to fibers that large temperature swings can cause, resulting in a better quality fiber, and therefore increased strength and yield of pulp.
While it is desirable to take advantage of the improvements compared to conventional cooks set forth above, under some circumstances it may still be desirable to run a digester with conventional processes, e.g. run it as an EMCC.RTM. digester. Therefore, it is desirable to construct a digester that can be switched over from the completely countercurrent process of the invention, to a conventional form of digester, such as an EMCC.RTM. digester.
According to the method of the present invention, cellulosic pulp is produced from comminuted cellulosic fibrous material utilizing an upright digester having a top and a bottom. The method comprises the following steps: (a) Continuously introducing comminuted cellulosic fibrous material entrained in cooking liquor into the top of the digester so that the material continuously moves downwardly in the digester. (b) Continuously withdrawing cellulose pulp from the bottom of the digester. And, (c) establishing a countercurrent flow between cooking liquor and material throughout the entire height of the digester between the material introduction at the top and the pulp withdrawal at the bottom, including continuously withdrawing liquid from the digester at various points and reintroducing the withdrawn liquid.
Preferably the digester is part of a two-vessel hydraulic system, which includes an impregnation vessel operatively connected to the inlet to the digester. In that case, step (c) is practiced in part by continuously withdrawing liquid from the top of the digester to a top extraction, recirculating a part of the liquid with any entrained material, to the impregnation vessel, and continuously removing a part of the withdrawn liquid from the digester-impregnation vessel loop (typically feeding it to a flash tank). The removed liquid has dissolved lignin therein (i.e. is "black liquor"), and is handled in the same way that black liquor is conventionally handled.
Also according to the invention, in order to prevent hot cooking liquors from entering the recirculation line back to the impregnation vessel, and thus reacting with incoming uncooked chips (which would cause the cooking chemical to be consumed before extraction could occur and thus result in low or no residual cooking chemical in the extracted liquor, and subsequent non-uniform cooking), a cooling circuit is provided at the top of the digester after the top extraction. The other liquids withdrawn from a mid-point of the digester, and the bottom of a digester, which are recirculated are heated so that cooking does take place below the cooling circulation.
In order to convert from the complete countercurrent continuous cook according to the invention to conventional cooking techniques, an upper mid-point extraction screen, between the top extraction and the mid-point extraction, can be maintained in place, cut off by a valve. When the valve is open, and a valve from the top extraction to the flash tank closed, conventional EMCC.RTM. treatment may be practiced. Under these circumstances, a valve in a white liquor line for adding white liquor to the bottom circulation can also be opened, or alternatively white liquor can be added to the bottom circulation even during complete countercurrent cook.
The invention also comprises a continuous digester including: An upright hydraulic vessel having a top and a bottom. An inlet for cellulose material entrained in liquid at the top of the vessel. An outlet for cellulose pulp at the bottom of the vessel. A top extraction, adjacent the top of the vessel, for withdrawing liquid from the top of the vessel. A top circulation loop connected to the top extraction and including a first conduit for removing a substantial volume of liquid from that withdrawn through the top extraction and utilizing it so that it is not reintroduced into the vessel, and a second conduit for recirculating liquid not removed, so that it returns to the impregnation vessel. A mid-point extraction for withdrawing liquid from the vessel. A mid-point circulation loop connected to the mid-point extraction and for reintroducing liquid extracted from the mid-point extraction so that it flows upwardly in the vessel. A bottom extraction for withdrawing liquid from the vessel. And, a bottom extraction loop connected to the bottom extraction for reintroducing liquid extracted from the bottom extraction so that it flows upwardly in the vessel.
The top circulation may include an in-line drainer from which the first and second conduits extend, and the digester may be in combination with the hydraulic impregnation vessel connected to the inlet to the vessel and to the second conduit. A cooling recirculation loop may be connected to the digester adjacent, but below, the top extraction, including the cooler. An upper mid-point extraction may be provided between the top extraction and the mid-point extraction, including an extraction conduit; and a first valve means is disposed in the first conduit, and second valve means is disposed in the upper mid-point extraction conduit. A control, such as a computer control, controls the first and second valve means so that when one is open the other is closed. A flash tank is connected to the first conduit and the upper mid-point extraction downstream of the valve means. The extractions typically include screens mounted within the digester.
It is the primary object of the present invention to provide an advantageous apparatus and method for continuous countercurrent cooking of cellulosic fibrous material to produce cellulose pulp, and to provide versatility for switching back from the completely countercurrent cooking to more conventional cooking techniques. This and other objects of the invention will become clear from the detailed description of the invention and from the appended claims.